Process for separating aromatic hydrocarbons



DISTILLATION STRIPPER EXTRACTOR COLUMN Nov. 20, 1962 F J. ZUIDERWEG ETA1.

PROCESS FOR SEPARATING AROMATIC HYDROCARBONS Filed 0G13. 23, 1959 THEIRATTORNEY United States Patent Oiice 33,065,167 Patented Nov. 20, 1962ware Filed Get. 23, 1959, Ser. No. 843,412 Ciairns priority, appiicationGreat Britain Mar. 24, 1959 9 Claims. (Cl. 20S- 311) This inventionrelates to a process for the extraction and recovery of aromatichydrocarbons from a liquid hydrocarbon mixture.

Several processes for the extraction and recovery of aromatichydrocarbons from liquid hydrocarbon mixtures have already beenproposed. Various high boiling, aromatic selective solvents have beensuggested, among these are diethylene glycol, dipropylene glycol, andsulfolane. Because of the light-heavy selectivity of these solvents, thehydrocarbons in the mixture are extracted in the following order: lightaromatics, heavy aromatics, light paraflns, and heavy paraflins. It hasdeveloped in the use of these high boiling solvents that when theprocess is operated to obtain a high yield of aromatics, contaminationwith light paraihnic material is usually experienced. A procedure forincreasing the purity is de? scribed in British Patent No. 739,200wherein a hydrocarbon mixture containing aromatic hydrocarbons isintroduced into a multi-stage extractor at an intermediate point and aglycolic, water-containing selective solvent for aromatic hydrocarbonsis introduced into one end of the extractor. At the same end a ratlinatepoor in aromatics and in solvent is withdrawn, while at the other end anaromatic-rich extract phase is withdrawn and introduced into a columnwhich is at a lower pressure than the extractor and wherein only part ofthe most volatile aromatics, together with non-aromatics of equivalentvolatility, are removed as the top products and after condensation arereturned as reflux to the extractor at the end from which thearomatic-rich extract is withdrawn. From the remainder of the extractphase the aromatics are separated from the solvent by distillation in adistilling column at atmospheric or subatmospheric pressure, directsteam being injected near the bottom of the distilling column. lt isstated in the example of the British patent that by using this processthe purity of the recovered aromatics may amount to 97.5% and that thisvalue may be increased to about 98% by using a more complex recoverysystem for the extract phase comprising three columns instead of two,and using two diiierent reflux streams instead of one single reduxstream. This addition of a further column together with all itsauxiliary equipment to the recovery system which is obviously a verycostly measure, clearly demonstrates the extreme rdillicultiesencountered in attempting to increase the products purity in extractionprocesses at the present type.

In this connection it should be noted that the speciiications fornitration grade toluene require a very high aromatics content, viz. ofat least 98.5% by volume (cf. ASTM specification 13841-).

It has now been found that by introducing certain changes in the processas described in British specification No. 739,200 and by adhering tocertain specic operating conditions, it is possible to obtain aromaticshaving a purity of 97.5-99% or even higher, while using a simplerecovery system that involves two columns only.

The invention will now be illustrated with reference to the accompanyingschematic drawing, wherein the sole gure is a process ilow diagram of apreferred embodiment of the improved process.

The combination of the following conditions has been found to give theaforesaid improved results:

(1) The solvent employed is a high boiling aromatic selective materialhaving an atmospheric (normal) boiling temperature within the range of225 to 295 C., and should contain not more than 2% by weight of water.Suitable solvents include diethylene glycol, dipropylene glycol,sulfolane, and mixtures thereof.

(2) The temperature in the extraction zone should be between and 155 C.

(3) The extract-phase from the extraction system should enter thestripping zone with a temperature between 140 and 155 C.

(4) The pressure in the stripping zone should be lower than the pressurein the extraction system, but not so low as to cause appreciable flashvaporization (i.e. vaporization by pressure reduction without heating)and should be at least 1.5 atm. abs.

(5) The top temperature in this stripping zone `should be between 140and 155 C. and the bottom temperature should be at least 190 C.

(6) The condensed top vapors from the stripping zone should be freedfrom substantially all water present as a second liquid phase beforebeing returned to the extraction system.

(7) The bottom product from the stripping zone should be cooled beforeexpansion to distilling zone pressure by at least 15 C. to a temperaturebetween 170 and 180 C.

(8) The pressure in the distilling zone should be below 0.5 atm. abs.

(9) The bottom temperature in the distilling zone should be at least C.,but should be at least 30 C. below the bottom temperature in thestripping zone.

(10) Direct steam should be injected into said distilling zone in orderto enable the hydrocarbons to be distilled off at not too high bottomtemperatures.

Thus, according to the present invention there is provided a process forthe extraction and recovery of aromatic hydrocarbons from a liquidhydrocarbon mixture containing one or more aromatic hydrocarbons, byintroducing the mixture into a counterflow multistage extraction system,introducing, at one end of the system, a high boiling, aromaticselective solvent having an atmospheric boiling temperature within therange of 225 C. to 295 C. and containing dissolved water; maintainingthe extraction system under pressure suicient to keep the flowingcontents liquid and maintaining the owing contents at an elevatedtemperature; withdrawing a railinate poor in aromatics and in solventfrom the same end of the system as that at which the solvent isintroduced; withdrawing an aromatic-rich extract phase from the otherend of the system, stripping said extract phase in a stripping zone toliberate a mixture of aromatic and nonaromatic hydrocarbons, saidstripping being effected at a pressure lower than that prevailing in theextraction system, condensing the so liberated vapors and returning thecondensate to the extraction zone; conducting the remainder of theextract phase to a distilling zone operating at a sub-atmosphericpressure, in which distilling zone separation is etfected betweenhydrocarbons a/nd solvent, with direct steam being introduced into saidzone; and returning substantially hydrocarbon-free solvent containingdissolved water to the extraction system. The temperature in theextraction system is maintained between 140 and 155 C. The solventsupplied to one end of the extraction system contains not more than 2%by weight of water. The extract-phase withdrawn from the extractionsystem is introduced into the stripping zone,`

at a temperature between 140 C. and 155 IC. without the occurrence ofappreciable flash vaporization. The stripping zone is operated at apressure of at least 1.5

atm. abs. at a top temperature between 140 C. and 155 C. and at a bottomtemperature of at least 190 C. The vapors from said stripping zone arecondensed and recycled to the extraction system after removingsubstantially all water present as a second liquid phase. The condensedvapors are introduced into the extraction system at the end at which theextract phase is withdrawn and/or at an intermediate point of theextraction zone the bottom product of the stripping zone is cooledbefore expansion to the distilling zone pressure, by at least C. to atemperature between 170 and 180 C. The distilling zone is operated at apressure of less than 0.5 atm. abs. and at a bottom temperature of atleast 150 C., with the bottom temperature being at least 30 C. lowerthan the bottom temperaure in the stripping zone.

The present process can be applied to feedstocks having a wide or anarrow boiling range. It is especially suitable for separating aromaticsfrom catalytically reformed gasolines, such as hydroformates andplatformates, or from fractions thereof. rihe feedstock should have anASTM nal boiling point of not higher than 220 C. Preferably the uppercutting point should be not higher than 160 C. When using a reformatefraction as starting material the upper cutting point shouldadvantageously be approximately the same as the upper cutting point ofthe feedstock for the reforming operation. The lower cutting point =ofthe feedstock should preferably be about 100 C.

When carrying out the process according to the invention the volumetricratio of solvent to hydrocarbon feed should be between 3:1 and 8:1,preferably between 4:1 and 6:1, whereas the ratio `of the amount of topproduct returned from the stripping zone to the extraction system to theamount of hydrocarbon feed should be between 0.2:1 and 0.7:1, preferablybetween 0.3:1 and 0.5:1.

Various high boiling solvents maybe used in the present process. Amongthese are diethylene glycol, dipropylene glycol and sulfolane.Diethylene glycol is preferred. The suitable solvents boil within thetemperature range of 225 C. to 295 C. The solvent used in the extractionsysteml may contain a small amount, not exceeding 2% by Weight, ofwater.

The extraction system should be a countercurrent multistage extractionsystem, e.g. a column containing packing material, or sieve plates, arotating disc contactor, a multiplicity of mixer-settler combinations,and the like. The number of theoretical stages should preferably be atleast 5.

The feed to the extraction system may be introduced as an intermediatepoint, but it is generally preferred to introduce it at or at least nearthat end of the extraction system at which the aromatic rich extractphase is withdrawn, because in that case the best compromise betweenproduct purity and recovery (yield) is usually realized. Suitable inletpoints are at said end of the extraction system (viz. at the firsttheoretical stage) or at the second theoretical stage, the latterembodiment being preferred because in that case the purity is distinctlyhigher whereas the recovery is `only slightly decreased. Feedintroduction at a plurality of points between the middle and the extractphase end of the system may be useful under specific circumstances.

The stream of hydrocarbons and solvent that is obtained as top productfrom the stripping zone and (after condensation and after removingsubstantially all water separating as a second liquid phase in thecondensing operation) is returned to the extraction system, should alsobe introduced at one or more points at or near the end of the extractionsystem at which the aromatic rich extract phase is withdrawn. It ispreferred to introduce this stream at `or near the end of the system(i.e. at the rst or second theoretical stage of the system) because inthese cases the best compromise between product purity and recovery isrealized. The recycle material may 4 in some instances be advantageouslyreturned to the ex.- traction system in a plurality of streams betweenthe middle and the extract phase end of the system. If necessary, therecycle stream may be vheated before entering the extraction system.

The temperature in the extraction system should be between and 155 C.There may be a certain temperature gradient over the system, providedthat the above limits are adhered to.

The extract phase leaving the extraction system `is introduced in thestripping zone, at or near its top, with a temperature between 140 and155 C., which implies that there will be no or substantially nointentional cooling of this stream between extraction system andstripping zone.

The stripping zone is operated at a pressure that is lower than thatprevailing in the extraction system but that is at least 1.5 atm. abs.The diierence in pressure in the extraction system and in the strippingzone should be not so large as to cause appreciable flash vaporization.In fact, if there is any vaporous material present in the extract phasewhen entering the stripping zone, the gravimetric ratio of this vaporousmaterial to the fresh feed to the extraction system should not exceed1:10. Preferably this ratio should be below 5:100, and in the mostpreferred embodiment no vaporous material will be present at all. Inthis zone the top temperature should be between 140 and 155 C., and thebottom temperature should be at least 190, the difference between topandV bottom temperature being at least 40 C. The top product containssubstantially all water and non-aromatic hydrocarbons present in theextract phase, and, in addition, some solvent and aromatic hydrocarbons.

In order to substantially reduce or prevent deterioration of the solventdue to lthe high bottom temperature in the 4stripping zone it isgenerally advisable that the solvent should contain a minor proportion,e.g. between 0.05 and 1% by weight of phenothiazine or of a substitutedphenothiazine. The use of these compounds for the purpose referred to isdescribed in -greater detail in the co-pending application Serial No.862,761 of Philip I. Garner, led December 30, 1959.

The bottom product leaving the stripping zone is cooled by at least 15C. to a temperature between 170 and 180 C., and then passed to thedistilling zone operating at a pressure below 0.5 atm. abs., preferably0.2 atm. abs. and at a bottom temperature of at least C., whichtemperature should always be at least 30 C. and preferably at least 40C. lower than the bottom temperature in the stripping zone.

Whereas the stripping zone will be operated without introduction ofdirect steam near or in the bottom part, direct steam should beintroduced into the distilling zone because otherwise the temperaturesrequired to obtain a substantially hydrocarbon-free solvent as thebottom product would be so high as to cause decomposition of thesolvent.

As a consequence of this introduction 4of steam in the ydistilling zonethe solvent leaving the distilling zone contains a certain amount (lessthan 2% by weight) of dissolved water.

lf necessary, there may be cooling of the solvent stream that isrecycled to the extraction system.

In the preferred embodiment of the present process the followingoperating conditions are adhered to:

Temperature in extraction system C, 150 Temperature of extract phase onentering stripping zone Top temperature in stripping zone C 145 Bottomtemperature in stripping zone C 195 Temperature of bottom stream fromstripping zone after cooling but before expansion to distilling zonepressure C 170 Top temperature in distilling zone C 85 Bottomtemperature in distilling zone C-.. 155

Pressure in extraction system atm. abs 5 Pressure in stripping zone atm.abs-- 1.8 Pressure in distilling zone atm. abs 0.2

Under these conditions the water content of the solvent leaving thedistilling column will be about 0.6% by weight.

It should be realized that the present process, in which there issubstantially no iiash vaporization between extraction system andstripping zone, a rather large temperature drop over the stripping zone,a distinctly higher bottom temperature in the stripping zone than in thedistilling zone, and a cooling between stripping zone and distillingzone, is fundamentally different from the approach of UK. patentspecification No. 739,200, already mentioned above, in which thetemperature drop over the stripping zone is rather small (as reflectedby statement that it is usually not necessary to add heat to thestripping zone by means of a reboiler), in which the main heat input inrecovery system is obviously in the bottom of the distilling zone andnot in that of the stripping zone, and in which there is obviously noexternal cooling between stripping zone and distilling zone.

As a consequence of this essentially different situation the presentprocess has the advantage that a greater proportion of the non-aromaticsstill present in the extract phase is removed in the overhead of thestripping zone, which results in a higher purity of the final extract ascompared with the previous process.

A liquid hydrocarbon mixture containing both aromatic and non-aromatichydrocarbons, is extracted in a multi-stage countercurrent extractor 1,which operates under pressure and at a temperature between 140 and 155C. For this purpose the feed is introduced into the extractor throughone or more of the feed inlet lines 2 to 5, whereas the selectivesolvent, containing dissolved water, is introduced into the extractor 1at or near its top through line 6.

The rainate phase, which contains only relatively small amounts ofwater, solvent and aromatic hydrocarbons, is withdrawn from the top ofthe extractor 1 through line 7 and further processed to removesubstantially all solvent present therein.

The aromatic-rich extract phase is withdrawn from the bottom of theextractor through line 8 and passes through a reducing valve (not shown)to the stripper column 10, the cooled extract phase being introduced ator near the top of the stripper. The pressure in column 10 is at least1.5 atm. abs., but lower than the pressure in extractor 1.

The stripper 10 is provided with a reboiler 11 .and is operated with afairly high temperature drop over the column. ln the stripper,separation is eliected into a top product containing some solvent, partof the aromatic hydrocarbons and most of the Water and non-aromatichydrocarbons present in the extract phase leaving the extractor 1, and abottom product that contains only a small amount of water and containssolvent and aromatic hydrocarbons and at most a minor amount ofnonaromatic hydrocarbons. The top vapors are passed through line 12 tothe condenser 13 and the resulting liquid passed to the settler 14,wherein separation in two layers, viz. a water-rich layer and a layerconsisting mainly of solvent and hydrocarbons, takes place. The waterlayer is removed via line 15, whereas the solventhydrocarbon layer ispassed through line 16 and heater 17, to one or more of the lines 18 to20, through which it enters the extractor in its lower part.

The bottoms product from the stripper is passed through line 21,provided with a cooler 33 and with a subsequent reducing valve (notshown) to distilling column 22. In column 22, which operates atsubatmospheric pressure, separation is effected into a top productcontaining aromatic hydrocarbons and substantially free 6 from solvent,.and into a bottom product, that is substantially free from aromatichydrocarbons.

The top vapors are withdrawn through line 23, provided with condenser 24and the condensate is partly Withdrawn via line 25, as the aromatic-richproduct, and partly passed through line 26 equipped with the settler26A, wherein separation in two layers, viz. a lower watererich layer andan upper layer consisting mainly of hydrocarbons, takes place. The upperlayer is recycled as reliux to column 22. The bottom product from thiscolumn is withdrawn through line 27 and returned to the extractorthrough line 6, if necessary after cooling in cooler 28. If desired,part of the bottom product may be withdrawn through line 29 forpurification or rejection, and fresh orpuried solvent may be introducedthrough line 30. Distilling column 22 is operated without a reboiler,hot steam being introduced into the bottom of this column via line 31and heater 32 to strip dissolved hydrocarbons from the descendingliquid.

lt should be realized that the drawing and its above description areschematic and that many auxiliary features such as valves and pumps,have not been represented or discussed.

We claim as our invention:

1. A process for the extraction and recovery of aromatic hydrocarbonsfrom a liquid hydrocarbon mixture containing aromatic and non-aromatichydrocarbons cornprising (l) introducing the mixture into a counterflowmulti-stage extraction system wherein the temperature is maintainedbetween and 155 C.; (2) introducing, at one end of the extractionsystem, a high boiling, aromatic selective solvent having a normalboiling temperature within the range of 225 C. to 295 C. and whichsolvent contains dissolved water in the proportion of not more than 2%by weight; (3) passing the solvent in countercurrent flow to thehydrocarbon mixture to obtain a solvent extract phase enriched inaromatics; (4) removing the extract phase from the extraction system andintroducing said extract phase at a temperature between 140 and 155 C.without appreciable flash vaporization into a stripping zone, which isoperated at a pressure of at least 1.5 atmospheres absolute and at a toptemperature between 140 C. and 155 C. and with a bottom temperature ofat least 190 C. to separate overhead a vapor phase enriched innon-aromatic hydrocarbons; (5) condensing the vapors from said zone andforming a first liquid phase consisting essentially of hydrocarbonsalong With a second liquid phase `and recycling them to the extractionsystem after removing substantially all water present as the secondliquid phase, with the condensed vapors being introduced into theextraction system between the intermediate point of the system and theend at which the extract phase is withdrawn; (6) withdrawing the liquidremainder of the extract phase from the stripping zone and cooling thewithdrawn extract phase before reduction in pressure by at least 15 C.to a temperature between 170 and 180 C., and (7) distilling the cooledextract phase in a distillation zone operated at a pressure of less than0.5 atmosphere `absolute and a bottom temperature of at least C. whichis at least 30 C. below the bottom temperature in the stripping zonewhereby an aromatic-rich fraction is separated as an overhead product.

2. A process in accordance with claim 1 wherein the aromatic selectivesolvent is selected from the group consisting of diethylene glycol,dipropylene glycol, sulfolane, and mixtures thereof.

3. A process according to claim l in which the feed mixture is acatalytically reformed gasoline, or a fraction thereof.

4. A process according to claim 1 in which the feed stock has an uppercutting point not higher than C.

5. A process according to claim 1 in which the feed to the extractionsystem is introduced near that end of e' the extraction system at whichthe extract phase is withdrawn.

6. A process according to claim 1 in Which the condensed vaporssubstantially free from Water are introduced near the end of theextraction systern at which the extract phase is Withdrawn.

7. A process according to claim 1 in which the extract phase isintroduced in the stripping zone near the top of said Zone.

8. A process according to claim 1 in which the Weight ratio of thevaporous material present in the extract phase on entering the strippingZone to the fresh feed to the extraction system is less than 5 100.

9. A process according to claim 8 in which no vapors are present in theextract phase on entering the stripping zone.

References Cited in the file of this patent UNITED STATES PATENTSSchumacker Dec. 15, Durrum Sept. 17, Weedrnan Oct. 9, Poienberger Aug.20, Findlay Sept. 10, Hawkins et a1. Oct. 8, Broughton Mar. 17, ScoeldSept. 22,

FORETGN PATENTS Great Britain Oct. 26,

1. A PROCESS FOR THE EXTRACTION AND RECOVERING OF AROMATIC HYDROCARBONSFROM A LIQUID HYDROCARBON MIXTURE CONTAINING AROMATIC AND NON-AROMATICHYDROCARBONS COMPRISING (1) INTRODUCING THE MIXTURE INTO A COUNTERFLOWMULTI-STAGE EXTRACTION SYSTEM WHEREIN THE TEMPERATURE IS MAINTAINEDBETWEEN 140 AND 155*C.; (2) INTRODUCING, AT ONE END OF THE EXTRACTIONSYSTEM, A HIGH BOILING, AROMATIC SELECTIVE SOLVENT HAVING A NORMALBOILING TEMPERATURE WITHIN THE RANGE OF 225*C. TO 295*C. AND WHICHSOLVENT CONTAINS DISSOLVED WATER IN THE PROPORTION OF NOT MORE THAN 2%BY WEIGHT; (3) PASSING THE SOLVENT IN COUNTERCURRENT FLOW TO THEHYDROCARBON MIXTURE TO OBTAIN A SOLVENT EXTRACT PHASE ENRICHED INAROMATICS; (4) REMOVING THE EXTRACT PHASE FROM THE EXTRACTION SYSTEM ANDINTRODUCING SAID EXTRACT PHASE AT A TEMPERATUE BETWEEN 140* AND 155*C.WITHOUT APPRECIABLE FLASH VAPORIZATION INTO A STRIPPING ZONE, WHICH ISOPERATED AT A PRESSURE OF AT LEAST 1.5 ATMOSPHERES ABSOLUTE AND AT A TOPTEMPERATURE BETWEEN 140*C. AND 155*C. AND WITH A BOTTOM TEMPERATURE OFAT LEAST 190*C. TO SEPARATE OVERHEAD A VAPOR PHASE ENRICHED INNON-AROMATIC HYDROCARBONS; (5) CONDENSING THE VAPORS FROM SAID ZONE ANDFORMING A FIRST LIQUID PHASE CONSISTING ESSENTIALLY OF HYDROCARBONSALONG WITH A SECOND LIQUID PHASE AND RECYCLING THEM TO THE EXTRACTIONSYSTEM AFTER REMOVING SUBSTANTIALLY ALL WATER PRESENT AS THE SECONDLIQUID PHASE, WITH THE CONDENSED VAPORS BEING INTRODUCED INTO THEEXTRACTION SYSTEM BETWEEN THE INTERMEDIATE POINT OF THE SYSTEM AND THEEND AT WHICH THE EXTRACTION PHASE IS WITHDRAWN; (6) WITHDRAWING THELIQUID REMAINDER OF THE EXTRACT PHASE FROM THE STRIPPING ZONE ANDCOOLING THE WITHDRAWN EXTRACT PHASE BEFORE REDUCTION IN PRESSURE BY ATLEAST 15* C. TO A TEMPERATURE BETWEEN 170* AND 180*C., AND (7)DISTILLING THE COOLED EXTRACT PHASE IN A DISTILLATION ZONE OPERATED AT APRESSURE OF LESS THAN 0.5 ATMOSPHERE ABSOLUTE AND A BOTTOM TEMPERATUREOF AT LEAST 150*C. WHICH IS AT LEAST 30*C. BELOW THE BOTTOM TEMPERATUREIN THE STRIPPING ZONE WHEREBY AN AROMATIC-RICH FRACTION IS SEPARATED ASAN OVERHEAD PRODUCT.